Yes I think mahou is spot on. I tried the multiple deletion using the media library after reading about it on here and didn't realise at first it only moves them to the trash. I think you can probably delete documents like saved web articles from it, but for books, the only way to permanently delete them is to do it individually via the MYK page.
I don't think you can actually delete them from the MYK page once they are in the trash of the media library, because the books I did that with are now no longer showing in the MYK page. You'd need to go back into the media library and restore them first. But I'm not about to do that because there is no way I am sitting wasting hours of my time painfully deleting one book at a time while having to wait for the page to refresh each and every time.
